1. Field of the Invention
The present invention relates to a control system for a steplessly variable power transmission and, more specifically to a hydraulic control system controlling a speed ratio (rotation speed of an input member/rotation speed of an output member) in the transmission under a certain driving condition of a vehicle.
2. Description of the Prior Art
There has been known a steplessly variable power transmission provided with a pulley mechanism as an engine power transmission mechanism having a primary pulley member connected with an engine and a secondary pulley member connected with vehicle wheels of which an effective diameter is changeable by controlling a hydraulic pressure therefor so that a speed ratio therebetween can be controlled. U.S. Pat. Nos. 4,152,947 (corresponding to Japanese Patent Publication No. 63-27592), 4,665,775, 4,671,138, 4,671,140, 4,673,377 and 4,796,489 all disclose controls for belt-pulley type steplessly variable power transmissions.
Japanese Patent Public Disclosure No. 60-53257 discloses a line pressure regulating valve actuated by employing a pilot pressure introduced thereinto for adjusting a line pressure introduced into respective hydraulic chambers for the primary and secondary pulleys and a speed ratio control valve of a pilot pressure action type for controlling an introduction of the line pressure into the hydraulic chamber for the primary pulley wherein the pilot pressures of the line pressure regulating valve and the speed ratio control valve are controlled by a solenoid valve providing a duty control.
With such a duty control, the following problems can occur. The viscosity of converter oil or hydraulic fluid increases as the temperature thereof is decreased. If the duty ratio of the solenoid valve providing pilot pressure at a low temperature of the hydraulic fluid is maintained at the same value as that at a normal temperature, the quantity of fluid drained by the solenoid valve is reduced because of the high viscosity of the hydraulic fluid. Therefore, the pilot pressure is unduly increased beyond a desirable value. This causes an unstable control of the pilot pressure; and thus an unstable speed ratio control.
In addition, a problem may occur in a control circuit for the solenoid valve. When shortcircuiting occurs in the control circuit, the solenoid valve is kept Off so that the pilot pressure is fixed at the maximum value. As a result, an effective control of the speed ratio cannot be accomplished.